Rail gun damage?

[Kristopher]

Those would have to be pretty huge thrusters. They have to basically put out the same amount of energy that the railgun round contains.

Newton's second law say that F = ma, i.e., momentum is mass times acceleration. If a railgun round of mass x is accelerated at rate a, it will contain xa momentum, which will shove the firing ship with that much force. The thruster will have to emit the same amount of force in the opposite direction to counteract this.

Which means you really don't want to in the thruster's exhaust plume.



This is a problem with all non-seeking weapons, be they cannons, railguns, laser batteries, or particle-beam weapons. And there is no real solution, short of time travel or telepathy.

Your target has a certain area that represents its possible future locations. The area is determined by the time-of-flight of your weapon from you to the target, the target's initial trajectory, and the target's engines (how much can it change its trajectory within the time-of-flight).

If this area is larger than the target silhouette, you probably will never hit the target.

Possible solutions include making your weapon home in on the target (despite the target's attempts to jam the homing mechanism), and area-effect warheads. Both of which work poorly with railguns.

If your target doesn't realize you've fired, he might not dodge the shot.

As for the recoil issue, as you say, F=MA. Isn't the ratio of mass between the projectile and the firing ship then an important issue? As most imaginable railgun or coilgun projectiles are miniscule in relation to the firing ship, shouldn't that offset a lot of the acceleration?

[Nyrath]

If your target doesn't realize you've fired, he might not dodge the shot.

Yes, but that will take some doing with a railgun. You insert the round encased in a sabot into the railgun. It strikes an arc with the highly charged rails, making a huge electrical explosion and showers of sparks. This explosion travels down the rails as the round accelerates.

If you want to hide this from the target, the railgun has to be inside of your ship. So the traveling explosion also happens inside of your ship.

And there is no good way to keep the target from noticing the red-hot railgun round streaking out of the gun muzzle and heading to a point right between your eyes.

As for the recoil issue, as you say, F=MA. Isn't the ratio of mass between the projectile and the firing ship then an important issue? As most imaginable railgun or coilgun projectiles are miniscule in relation to the firing ship, shouldn't that offset a lot of the acceleration?

Yes. If the mass difference is great enough, the ship will not undergo much acceleration. Unless the railgun round is traveling at hypervelocity. Then all bets are off.

Keep in mind that there is a ship propulsion system called a "mass driver", which is basically using a railgun for rocket propulsion.

If you want to calculate it:

As = (Mr * Ar) / Ms
where:
As = acceleration ship experiences (meters per second)
Mr = mass of railgun round (kilograms)
Ar = acceleration railgun round experiences (meters per second)
Ms= mass of ship (kilograms)

example:
The space dreadnaught Obnoxious has a mass of 13,500,000 kilograms (the same as a Russian Oscar class submarine), and its railgun fires 18 kilogram rounds.

If the rounds travel at 6000 meters per second (about the upper limit of current railgun technology), they will make the Obnoxious recoil at a speed of 8 millimeters per second, or a pathetic 0.0008 g.
It will strike the target with a force of 3.2 x 10^8 joules, about the same as a Battleship Iowa 16 inch shell with 54 kg high explosive charge (according to the Boom table).

However, if the rounds travel at 14% the speed of light (where relativity becomes a factor), each round will make the Obnoxious recoil at a speed of 56 meters per second, or a whopping 5.7 gs!!
It will strike the target with a force of 1.6 x 10^16 joules, about 3.5 megatons, or the same as the Barringer Meteor Crater

[Cancer]

Recoil: There is recoil when a railgun is fired. In space, where there is no friction, it can affect the ship's trajectory (remember, firing an autocannon deorbited an Almaz station). My solution: Fire thrusters to counteract the recoil at the same time the guns are fired.

Another possibility is to make your railguns "recoilless" in the same way existing RCL weapons are recoilless. Include a reaction-mass projectile (doesn't have to be the same mass, probably better if it's more massive) simultaneously launched in the opposite direction. I.e., your railguns are "double barreled" with the second one pointing the opposite way of the first. It means you're cutting your launcher efficiency (KE of weapon projectile per unit of supplied power) by of order 50%, but it solves some other problems elegantly. (It's "of order" 50% because what you want is the reaction projectile to have equal but opposite momentum, not KE, as the warhead projectile.)

Just make sure no friendlies are standing behind when you fire....

If you are using real physics, another serious problem happens if your railgun is mounted in any way except fixed directly on the momentum axis. If you have the gun in a steerable turret, there's going to be a bunch of angular momentum deposited into the ship when you fire, that is, you're going to make your ship spin. Even a pair of coaxially-mounted guns will do this unless their firing is carefully synchronized; if they have the same rate of fire but are out of sync you'll wag the nose back and forth with a series of alternating jolts in yaw. How bad this effect is depends on the comparison of the mass of the ship and the momentum of the projectiles. I think the whole point of a simple kinetic-kill shot-type weapon is that the projectile's impact is substantial compared to the target, so assuming combatants of comparable mass, the firer's momentum is going to be altered if the railgun is at all effective.

Now, you could turn that into a feature rather than a bug, assuming an adequately high rate of fire; your fire control system could take into account the recoil-driven back-rotation of the firing platform and use that to deliver a preplanned pattern of projectiles, sweeping out an arc in the sphere centered on the ship's center of mass. I'd have to do some numerical experiments to see how that might work.

[Kristopher]

And there is no good way to keep the target from noticing the red-hot railgun round streaking out of the gun muzzle and heading to a point right between your eyes.

Just how big do we expect the projectiles to be? I keep thinking of something smaller than a roll of dimes, based on what I've read. Is a projectile that small going to be noticed at a range of, oh, 100 km?

[Nyrath]

Just how big do we expect the projectiles to be? I keep thinking of something smaller than a roll of dimes, based on what I've read. Is a projectile that small going to be noticed at a range of, oh, 100 km?

Yes.

For one thing, it will be glowing red-hot, so it will show up on an infrared scan like a sore thumb.

For another thing, military spy satellites have a resolution of down to about 5 centimeters. From orbit.

And for a third thing, the target will be able to see your gun turrets, so they just have to keep a sharp eye on the gun muzzles, watching to see if anything emerges.

[Kristopher]

Yes.

For one thing, it will be glowing red-hot, so it will show up on an infrared scan like a sore thumb.

For another thing, military spy satellites have a resolution of down to about 5 centimeters. From orbit.

And for a third thing, the target will be able to see your gun turrets, so they just have to keep a sharp eye on the gun muzzles, watching to see if anything emerges.

Do those military spy sats take video from orbit at 5cm resolution? And will there be enough people or computer power on board a ship to watch for moving objects effectively a few pixels in size, over the entire field of view?

[tkdguy]

Another variable to consider is whether the target be able to respond in time. If the ship can't accelerate or change course quickly enough to avoid the sabot altogether, it can still be hit.

Would it be possible to "fake out" another ship?

[Wyrm Ouroboros]

This is in space, am I correct??

Not meaning to offend you, Nyrath, but where the heck are you getting any of your ideas from? I mean, don't get me wrong, your math is great, but this 'causes an explosion' crap is, well, crap; the primary side effect of modern-day mass accelerators -- railguns -- is a short-range EMP burst out the front end due to the electromagnetic acceleration of a slug of steel. No continuing electrical explosion; that isn't how the damn thing works. No 'red-hot slug'; it's a piece of steel suspended in and sent down a tube by computer-controlled electromagnets that takes it from 0 to a zillion m/s in the time it takes to sneeze.

Does this affect the ship/base? Yes; the back-thrust from accelerating a 50g piece of metal to Mach 25 is considerable, and yes, it may not do much more to a 250 tonne ship than nudge it off-course, but the firing still has to be taken into account -- like A-10 Warthog pilots have to, especially if you're going for broke. But if it's a ship, then the ship's thrusters are already set to move around 250 tonnes; adjusting for weapons-fire is going to be computed into the ship's piloting equations.

The question of whether or not the target ship can dodge, however, requires a bit of forethought. First, does the target know they've been fired upon? Second, how long does it take the slug to get from Attacker to Target? Third, if #1 is 'no', can they detect the round, however big it is, in time to pilot the ship out of the way? In space, once the round is fired, it's going to go down that line for quite a while, until/unless something happens, e.g. gravity effects, etc. Presuming a low-space-tech environment (which I think we are, since we're still using mass weapons), I'd imagine multiple 'likely trajectory' shots instead of just one round getting spat out. "At the speed they're going now, they can only go in these directions. Lay fire down on this path; they'll likely get hit by at least one of 'em ..."

[tkdguy]

I was thinking about firing multiple shots in one barrage. Maybe the target ship can dodge them all; maybe it will dodge some and get hit by the rest.

[Lawnmower Boy]

This is in space, am I correct??

Not meaning to offend you, Nyrath, but where the heck are you getting any of your ideas from? I mean, don't get me wrong, your math is great, but this 'causes an explosion' crap is, well, crap; the primary side effect of modern-day mass accelerators -- railguns -- is a short-range EMP burst out the front end due to the electromagnetic acceleration of a slug of steel. No continuing electrical explosion; that isn't how the damn thing works. No 'red-hot slug'; it's a piece of steel suspended in and sent down a tube by computer-controlled electromagnets that takes it from 0 to a zillion m/s in the time it takes to sneeze.....

Nyrath links to his website in his sig. It's well worth exploring. And he'll be along to make his case soon, no doubt. In the meantime, it seems pretty obvious a priori (Second Law of Thermodynamics and all that) that an object accelerated by a railgun will be heated as a byproduct. The "electromagnetic explosions" presumably refer to spikes in the electromagnetic flux produced for the same reason, which may be inferred to be the result of the action of real railguns having a periodic function.

[Kristopher]

This is in space, am I correct??

Not meaning to offend you, Nyrath, but where the heck are you getting any of your ideas from? I mean, don't get me wrong, your math is great, but this 'causes an explosion' crap is, well, crap; the primary side effect of modern-day mass accelerators -- railguns -- is a short-range EMP burst out the front end due to the electromagnetic acceleration of a slug of steel. No continuing electrical explosion; that isn't how the damn thing works. No 'red-hot slug'; it's a piece of steel suspended in and sent down a tube by computer-controlled electromagnets that takes it from 0 to a zillion m/s in the time it takes to sneeze.

Nyrath knows his stuff fairly well, which is why I'm asking him for clarification instead of assuming he's wrong.

I'm still wondering if coilguns aren't a better option, as IIRC they don't involve sending a current through the projectile.

And that's where I'm guessing the heating of the projectile comes from -- the current that has to be passed through it in a railgun, because it's part of the circuit.

[Nyrath]

Do those military spy sats take video from orbit at 5cm resolution? And will there be enough people or computer power on board a ship to watch for moving objects effectively a few pixels in size, over the entire field of view?

If they wanted to take video, I see no reason why not. And even if there was a technical reason that orbital video at 5 cm resolution is not currently feasible, I'm sure it will be possible by the time there are interplanetary warships. After all, a mere fifty years ago any kind of satellite at all was beyond the state of the art. What will the state of the art be fifty years from now?

Enough computing power? I think so. There are computers currently available today that are the size of a pack of cigarettes. You could have a room full of these, and have twenty or thirty watching each of the enemy's gun turrets.

[Nyrath]

Not meaning to offend you, Nyrath, but where the heck are you getting any of your ideas from? I mean, don't get me wrong, your math is great, but this 'causes an explosion' crap is, well, crap; the primary side effect of modern-day mass accelerators -- railguns -- is a short-range EMP burst out the front end due to the electromagnetic acceleration of a slug of steel. No continuing electrical explosion; that isn't how the damn thing works. No 'red-hot slug'; it's a piece of steel suspended in and sent down a tube by computer-controlled electromagnets that takes it from 0 to a zillion m/s in the time it takes to sneeze.

Well, maybe I was being overly dramatic , but have you seen these images?
http://thetension.blogspot.com/2008/...rlds-most.html

Keep in mind that we are talking about railguns, not coilguns. With railguns, the rails strike an arc with the sabot. A tremendous amount of head is generated. So much so that the rails have to be replace after a few shots due to severe erosion.

Does this affect the ship/base? Yes; the back-thrust from accelerating a 50g piece of metal to Mach 25 is considerable, and yes, it may not do much more to a 250 tonne ship than nudge it off-course, but the firing still has to be taken into account -- like A-10 Warthog pilots have to, especially if you're going for broke. But if it's a ship, then the ship's thrusters are already set to move around 250 tonnes; adjusting for weapons-fire is going to be computed into the ship's piloting equations.

I don't think you understand. In theory you can use a spray can of underarm deodorant as a ship thruster, it will just take a few days to change the ship's orientation a few degrees. By the same token, a thruster rated to move a 250 tonne ship might still take a day or so to stop your ship from spinning like a top due to the last railgun firing.

It all depends upon the specifics: how massive is the railgun round, how fast is it moving, how many rounds are expended, how massive is the ship, and how much thrust the attitude jets can produce.

The question of whether or not the target ship can dodge, however, requires a bit of forethought.

It does not have to. If you are under fire from an enemy, you do random evasive maneuvers just on general principles. You don't sit quietly until you see the shot.

First, does the target know they've been fired upon?

Yes. There is no way to hide the firing of the slug. I take it you've never read the page on my website explaining why the experts I've consulted have concluded that there ain't no stealth in space. If nothing else, the weapon turret will light up like a strobe light in the infrared spectrum due to waste heat.

Second, how long does it take the slug to get from Attacker to Target? Third, if #1 is 'no', can they detect the round, however big it is, in time to pilot the ship out of the way? In space, once the round is fired, it's going to go down that line for quite a while, until/unless something happens, e.g. gravity effects, etc. Presuming a low-space-tech environment (which I think we are, since we're still using mass weapons), I'd imagine multiple 'likely trajectory' shots instead of just one round getting spat out. "At the speed they're going now, they can only go in these directions. Lay fire down on this path; they'll likely get hit by at least one of 'em ..."

The people I know who have made space combat simulations see such weapons more as a way to create "terrain" in space. You fire wads of shots in certain directions to herd your target the way you want it to go (usually in front of your main guns). The railgun shots deny certain vectors to the target (at least if they want to avoid being hit), so they are forced to move where you want.

[Nyrath]

I'm still wondering if coilguns aren't a better option, as IIRC they don't involve sending a current through the projectile

Yes, every expert I've talked to say coilguns are superior. It's just that they are orders of magnitude harder to make.

The arc you strike with a railgun causes severe rail erosion. In current models you have to replace the rails after a few shots. On the other hand, we have close to weapons grade railguns now.

Coilguns have no such erosion. However, since each coil has to reverse its polarity as the projectile passes by, you need power switches capable of handling huge power loads. They are still trying to invent a switch suitable for a weapons grade coilgun, currently available switches are not good enough.

Coilguns will still have waste heat because of the second law of thermodynamics, but probably not anywhere near the waste from a railgun.

[megaplayboy]

Well, I suppose the analogy would be to jet fighters firing cannon at each other. The operative range of such weapons, relative to the speed of the jets is fairly small...if a jet is moving at 300m/sec (high subsonic speed) and the effective range of the shells is perhaps 1000 m, then transit speed is maybe 2-3 seconds.

So, if we posit that for mass driver weapons, then the maximum effective range is about 2-3 seconds travel time. An effective system would take into account all the possible vector shifts a target could make in that time, and bracket them with multiple mass drivers...so let's say an effective system has 20-40 "barrels" bracketing a fire zone in order to maximize chances to hit.

I remember reading somewhere that the practical speed limit for a mass driver is probably around 150km/sec. Applying that to the scenario gives an effective range of 300-500 km perhaps. At those velocities the projectiles could be fairly small, say in the 20-30mm range.